1. Field of the Invention
The present invention relates to a Mach-Zehnder type optical modulator and a transmitter used in optical communication.
2. Description of the Related Art
Optical waveguide devices employing an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO2) are formed by thermally diffusing a metal film on a portion of a crystal substrate or effecting proton exchange in a benzoic acid after patterning to form an optical waveguide followed by provision of a signal electrode near the formed optical waveguide. A Mach-Zehnder type optical modulator having a branching interference type optical waveguide structure is one such optical waveguide device that employs an electro-optic crystal.
Two-valued modulation modes where a signal level is either 0 or 1 are commonly used in communication. However, in recent years, there has been growing development of multi-valued modulation modes using three values or four values to increase transmission capacity. Among multi-valued signals are signals that provide multiple values to an output intensity of light, a phase of light, and a combination thereof. Such an example is presented in Japanese Patent Application Laid-Open Publication No. 2005-221874.
Methods of using a Mach-Zehnder type optical modulator to provide multiple values to an output intensity of light, include a method of applying an electric field to one of the parallel optical waveguides by using a multi-valued electrical signal subjected to wave combination by an electrical coupler to generate a multi-valued signal light and a method of applying an electric field to each of the two parallel optical waveguides by independent electrical signals to generate a multi-valued signal light. Since the method of applying an electric field to each of the two parallel optical waveguides by using independent electrical signals does not require an electrical coupler, electrical signal loss and band deterioration does not occur, thereby making this method advantageous in cost and size.
FIG. 21 illustrates a structure of a conventional optical modulator. A conventional optical modulator 2100 includes an incident optical waveguide 2101, a pair of optical waveguides 2102a and 2102b, a pair of signal electrodes 2103A and 2103B, and an exit optical waveguide 2104. Light that enters from the incident optical waveguide 2101 diverges and is transmitted through the optical waveguide 2102a and the optical waveguide 210b. The light transmitted through the optical waveguide 2102a and the optical waveguide 2102b interfere with each other in the exit optical waveguide 2104 and is transmitted as a signal light.
The signal electrode 2103A and the signal electrode 2103B are arranged along the optical waveguide 2102a and the optical waveguide 2102b. For example, when a Z-cut crystal substrate is used, the signal electrode 2103A and the signal electrode 2103B are arranged directly above the optical waveguide 2102a and the optical waveguide 2102b, respectively. When electrical signals are input to the signal electrode 2103A and the signal electrode 2103B and a voltage is applied, an electric field in the direction of the Z-axis varies the refraction indexes of the optical waveguide 2102a and the optical waveguide 2102b. 
The optical modulator 2100 controls electrical signals at the signal electrode 2103A and the signal electrode 2103B to produce a phase difference in the light respectively transmitted through the optical waveguide 2102a and the optical waveguide 2102b. For example, when voltages of +VΠ/2 and −VΠ/2 are applied to the signal electrode 2103A and the signal electrode 2103B respectively, the phase difference between the light transmitted through the optical waveguide 2102a and the optical waveguide 2102b becomes 180°, and the output intensity of the signal light transmitted from the exit optical waveguide 2104 becomes 0.
FIG. 22A illustrates a view of a first example of an optical electric field of each optical waveguide in a conventional optical modulator when both electrical signals in the signal electrode 103A and the signal electrode 103B are ON. In this example, the phase difference between light A transmitted through the optical waveguide 2102a and light B transmitted through the optical waveguide 2102b becomes 180°, and the output intensity of the signal light transmitted from the exit optical waveguide 2104 becomes 0.
FIG. 22B illustrates a view of a second example of an optical electric field of each optical waveguide in the conventional optical modulator when an electrical signal in the signal electrode 103A is ON and an electrical signal in the signal electrode 103B is OFF. In this example, the output intensity of a signal light C transmitted from the exit optical waveguide 2104 is ⅓, and the phase is −71°.
FIG. 22C illustrates a view of a third example of an optical electric field of each optical waveguide in the conventional optical modulator when an electrical signal in the signal electrode 103A is OFF and an electrical signal in the signal electrode 103B is ON. In this example, the output intensity of the signal light C transmitted from the exit optical waveguide 2104 is ⅔, and the phase is +48°.
FIG. 22D illustrates a view of a fourth example of an optical electric field of each optical waveguide in the conventional optical modulator when both electrical signals in the signal electrode 103A and the signal electrode 103B are OFF. In this example, the phase difference between light A transmitted through the optical waveguide 2102a and light B transmitted through the optical waveguide 2102b is 0, the output intensity of the signal light C transmitted from the exit optical waveguide 2104 is 1, and the phase is 0. Combining an input signal to the optical waveguide 2102a with an input signal to the optical waveguide 2102b in this manner enables four values for the output intensity of the signal light C transmitted from the exit optical waveguide 2104.
However, in a conventional optical modulator, since the phase of the signal light C transmitted from the exit optical waveguide varies according to modulation of the intensity, a wavelength chirp occurs in the signal light C, thus changing the wavelength of the signal light C. Therefore, the waveform deteriorates due to wavelength dispersion during transmission causing difficulty with demodulation on the receiving-side.
When intensity modulation by the conventional optical modulator is combined with phase modulation to carry out larger-capacity multi-valued modulation, the phase of the signal light C transmitted from the exit optical waveguide varies according to the intensity modulation, and hence this varying component turns to noise in the phase-modulated signal causing demodulation on the receiving-side to be difficult.
To solve these problems, it is an objective of the present invention to provide an optical modulator that, in generating a multi-valued signal light, reduces wavelength chirp and facilitates demodulation on the receiving-side, and provide a transmitter to which this optical modulator is applied.